The objective of this proposal is to identify biochemical alterations that play a critical role in ischemic brain damage. Our primary effort is directed at the role of lactacidosis in cellular damage. We hypothesize that there is a sharp threshold for brain pH (and/or lactate), at which restoration of high-energy phosphates is prevented even if the tissue is reperfused. Thus, the specific aims of this proposal are (i) to determine whether such a threshold exists by correlating tissue pH and lactate directly with energy state, (ii) to quantitate the critical levels of pH and lactate in various regions of brain, (iii) to determine the metabolic sites of inhibition using measurements of tissue metabolites and enzyme activities, and (iv) to modify the threshold level of lactacidosis by restricting lactate production and by increasing the buffer capacity of the tissue. Two models of focal ischemia in mouse brain will be employed: first, occlusion of the middle cerebral artery and second, occlusion of one carotid artery combined with systemic hypoxia. Both of these models have been developed and characterized in our laboratory and are suitable for the investigation of transient focal ischemia and extended recovery (1 week). Regional alterations of metabolite levels, pH (using the creatine kinase equilibrium), and tissue enzymes will be determined in microscopic samples (10-50 ng) using quantitative microchemical techniques in freeze-dried tissue. Further, the biochemical alterations will be correlated directly with histologic change in adjacent sections of frozen brain. Finally, exogenous agents, such as Tris buffer or dichloroacetate, will be used to modify the ischemic lactacidosis.